This invention relates to a high efficiency impeller for mixing, blending and agitating liquids and suspensions of solids in liquids.
The measure of the efficiency of a mixing impeller is its ability to create more liquid motion for a given amount of energy expended. Chemical processes may be described by a defined level of mixing processing required. A high efficiency impeller is capable of providing the required process performance with less power, or in less time, or with an impeller of smaller size as compared to an impeller of less efficient design.
Usually bulk fluid velocity and a high level of conversion of the power into axial fluid flow are factors which indicate efficient impeller performance. An efficient impeller is usually one which has a high degree of axial flow (as compared to rotational and radial flow). This is flow which spreads less, and which permits the impeller to be placed a greater distance from the bottom of the mixing vessel.
A higher impeller efficiency has the secondary advantage of permitting a decrease in shaft length, thus reducing the cost of the shaft and inherent instability problems found with greater shaft lengths. Alternatively, a lighter weight impeller of the same or better efficiency permits the use of longer shaft lengths, since the critical speed limits the shaft length, and the critical speed for an impeller is inversely proportional to the square root of the impeller weight.
Also, the benefits of greater efficiency include the ability to use scaled down or smaller drive components, or permit an increase in processing rate with the same components.
An important consideration in the design of such an impeller is the amount of material in the hub and blades to carry the torque and thrust. Another consideration is the adaptability of the design to "scale" in size between small impellers of only a few inches in diameter to large units, which may measure 20 feet or more in diameter.
The amount of material, or at least, the ability to reduce critical materials, is important in the production of competitive impellers. In certain applications, it is necessary to form the impeller components of relatively high cost and/or critical materials, and in very large sizes, the material cost may become the most significant factor in the cost of the impeller.
As previously noted, the ability of the design to be scaled up (or down) while maintaining its performance is an important practical consideration. The ease of scaling is also a factor. Also important is the ability to make all the impeller components, especially blades, with the same bends, chamfers, and angles regardless of size. This consideration favors blades made of flat sections or at least sections which are free of critical radii which necessarily change with size.
A successful impeller design which meets many of the above parameters is known as the HE-3 of Chemineer, Inc., the assignee of this application. This impeller uses three equally-spaced blades formed of approximately rectangular flat plates, with a single camber-inducing bend extending span-wise from a point on the leading edge at about a 50% span station, to a point on the blade tip somewhat forward of the chord center. The blade portion forward of the bend is turned downwardly about the bend line through an angle of about 20.degree.. The blade, at the root, is set on the support hub at a pitch angle of about 30.degree..
The HE-3 impeller meets may of the design criteria which are considered to be important to the commercial success of an agitating impeller, including the use of flat plate material formed or shaped with simple bends, to provide ease of scaling. It also exhibits desirably high axial flow characteristics. However, the blade design of the HE-3 impeller requires the use of relatively thick or heavy plate material, to provide sufficient beam strength at the root or hub end to support the bending and twisting loads on the blade. In the commercial embodiment, the hub, itself, at the blade attachment, is also reinforced by ribbing to augment the strength of the blade-conforming attachment boss.
There accordingly is a need for an agitation impeller which incorporates the design efficiencies of the HE-3 impeller, as outlined above, with less material weight and greater strength. Desirably, such an impeller should have performance characteristics which equal or exceed those of the HE-3.